1. Field of the Invention
The invention pertains to the field of keratoplasty and, more particularly, to the operation of a device configured to treat one or more eye disorders by applying energy in multiple steps to cause corrective reshaping of an eye feature.
2. Description of Related Art
A variety of disorders, such as myopia, hyperopia, and presbyopia, involve refractive disorders in the eye. For example, with myopia, the shape of the cornea causes the refractive power of an eye to be too great and images to be focused in front of the retina, affecting visualization of far objects. Meanwhile, hyperopia is a refractive condition characterized by a flattened corneal curvature or short eyeball that focuses light to a point behind the retina and prevents visualization of near objects.
In addition to glasses and contact lenses, invasive surgical procedures, such as laser-assisted in-situ keratomileusis (LASIK), may be employed to reshape the cornea to treat disorders, such as myopia and hyperopia. Such surgical procedures, however, typically require a healing period after surgery. In addition, such surgical procedures may result in complications, such as problems related to the lamellar flap or dry eye syndrome caused by the severing of corneal nerves. Moreover, LASIK is an ablative, subtractive procedure and, as such, is not ideal for treating hyperopia, because steepening the cornea to treat hyperopia requires the removal of a substantial amount of peripheral tissue.
Conductive keratoplasty (CK), the current surgical standard of care for hyperopia treatment, is an invasive procedure that involves applying low-level, radio frequency (RF) energy through needle electrodes placed in a circular pattern on the outer part of the cornea. The RF energy causes corneal fibers to shrink where applied, causing central steepening of the cornea. CK does not necessitate tissue removal but suffers from limits to the predictability of the outcomes and the durability of the results, in addition to the possibility of astigmatism induction from improper electrode placement.
Unlike hyperopia and myopia which are refractive dysfunctions caused by eyeball geometry and corneal shape, presbyopia is a refractive disorder that is caused by age-related changes in the crystalline lens and results in the loss of accommodation, i.e., an impaired ability to focus on near objects. In addition to glasses and contact lenses, three main surgical methods exist for the treatment of this disorder: (1) multifocal intraocular lenses (MIOLs), (2) monovision treatment employing LASIK or CK, and (3) multifocal LASIK (PresbyLASIK). The latter two are variations on refractive surgeries in current practice, while the former is the replacement of the natural lens with an artificial alternative.
Monovision treatment provides both near and distance vision by adapting one eye for distance, usually the dominant eye, and a second eye for near vision. Both LASIK and CK may be employed for monovision treatment. The monovision approach is not without limitations, however, as not all patients tolerate having each eye focused on a different plane. Indeed, it is recommended that patients undergo careful personality screening and experience simulated monovision with contact lenses before proceeding with refractive surgery. Those who do get the treatment have a 60-70% success rate with the main limitations being problems with night driving, impairment of stereopsis, and inability to focus on objects at an intermediate distance (e.g., computer monitors). These limitations have motivated the development of multifocal correction techniques, with the aim of preserving binocular vision.
The most common refractive surgery technique for bilateral multifocal correction is PresbyLASIK, a modified version of LASIK which treats presbyopia by creating both near and distance vision in a single cornea. Originally, multifocal laser ablation used photorefractive keratectomy (PRK), however, few modern approaches use this surface ablation technique anymore, with one reviewer suggesting that this is most likely due to complications with epithelial regrowth. LASIK avoids these compensatory reactions. Three different LASIK ablation profiles that induce both near and distance vision are described in the literature, each with their own advantages and disadvantages.
Transitional PresbyLASIK is characterized by an intentional decentration of the near vision zone to create a pattern reminiscent of a bifocal spectacle. However, the technique was not widely accepted by the surgical community as it required the creation of a significant vertical coma in the cornea and most surgeons were not comfortable with this approach.
Central PresbyLASIK steepens the center of the cornea to provide near vision, while flattening the periphery to provide distance vision. The ablative nature of LASIK makes this approach particularly attractive as it requires the removal of the least amount of tissue. However, this technique is prone to inducing corneal aberrations.
Peripheral PresbyLASIK produces the inverse zonal arrangement with distance vision through the central cornea and near vision through the periphery. This method potentially provides the greatest amount of pseudoaccomodation. However, because LASIK is subtractive, a significant amount of corneal tissue must be removed.
Moreover, a significant drawback of current multifocal treatments such as PresbyLASIK is the discontinuity of corneal curvature at the boundary of the near and distance vision zones. This results in instantaneous differences in refractive power that can cause visual aberrations such as starbursts and halos, adversely affecting night vision. Another drawback is the absence of an intermediate distance vision zone, resulting in an inability to focus on objects at an intermediate distance.